(1) Field of the Invention
This invention relates to drills and drill bits and particularly to the construction of split point drill bits with improved efficiency in the cutting of holes in the work piece.
(2) Discussion of the Prior Art
In the manufacture of split point drill bits for drilling holes in metal, wood, plastic, aluminum and castings, the drill bits usually include four cutting edges, two main or primary cutting edges plus two secondary cutting edges. The two main cutting edges of a split point drill bit are a function of the shape of the grinding wheels that grind helical flutes, the pitch of the helical flutes, and the point angle. The two secondary cutting edges are created by what is commonly referred to as a split point operation. In this operation, a main relief surface of the split point drill bit is relieved through an additional grinding operation creating the secondary cutting edges as well as a secondary relief surface.
Split point drill bits are provided with self-centering capabilities, but the self-centering capabilities of split point drill bits are limited due to the fact that all of the vibrations and the unsoundness of a drill chuck are fully transferred to the point of the drill bit as long as the drill bit is not in the material with its full circumference. This causes the holes drilled to be triangular shaped and over-sized. However, once the full diameter of the drill bit has penetrated the material to be cut, the tip of the drill bit then acts as a bearing providing stability and reducing vibrations. For a good quality hole it is necessary to reduce the initial vibrations by providing stabilization as early as possible. This can be done by pilot points such as the pilot points for the self-centering drill bit taught in U.S. Pat. No. 4,968,193 which teaches a pilot point with a diameter much smaller than the outer diameter of the drill bit wherein this pilot point provides stabilization within a fraction of an inch drilled into the material in which the drill bit is placed.
Another disadvantage of the split point drill bit is that the cutting velocity vectors along the two main or primary cutting edges point in different directions. The cutting velocity vector is always perpendicular to a line drawn from any point of the main cutting edges through the center of the drill bit. These cutting velocity vectors govern the chip flow velocity vectors and hence govern the direction of the chip flow. This means that the chip flow direction varies all along the radius of the main cutting edges. And, different chip flow directions mean that additional energy is consumed to tear the chips apart since the chips can only flow in one direction. Therefore, there is a particular need to provide split point drill bits which provide energy efficient drilling which is particularly useful in cordless drills.